Synthesis and magnetic characterization of (Tl_0.5Pb_0.5)Sr₂Ca₂Cu₃O₉ and Tl₂Ba₂CaCu₂O₈ bulk superconductors

Nearly single-phase bulk superconducting (Tl_0.5Pb_0.5)Sr₂Ca₂Cu₃O_9-δ (hereafter referred to as Tl, Pb-1223) samples were prepared readily from calcium- and copper-rich starting compositions of (Tl_0.6Pb_0.5)Sr₂Ca_2+zCu_3+zO_y (z = 0.4-0.5). The starting compounds were heated in a crimped gold tube at 850°C for 2 h in air with one intermediate grinding, and quenched in liquid nitrogen. The T_c was found to be 108 K (measured as the diamagnetic onset temperature) for the "as-synthesized" samples. The crystal symmetry was found to be tetragonal with lattice parameters a = 3.807 (2) Å, and c = 15.24 (1) Å, similar to the literature values. Magnetic characterization were conducted by using a DC-SQUID magnetometer. From the magnetization measurements, the intragrain critical-current density, J_c, was calculated to be 6 × 10⁵ A/cm² at 77 K and a field of 0.05 T. Flux pinning characteristics were investigated for Tl,Pb-1223 samples and the results are compared with that of Tl₂Ba₂CaCu₂O₈ (Tl-2212). The irreversibility fields and bulk flux-creep activation energies for Tl,Pb-1223 samples are found to be higher than for Tl-2212 at all investigated temperatures. The flux-line densities and the activation energy per flux are reported for the first time and reveal an increased defect density (serving as pinning centers) for Tl,Pb-1223 samples as compared to that for Tl-2212 samples at 35 K. These features are attributed to the three-dimensional behavior of Tl,Pb-1223 samples when compared to the two-dimensional (highly anisotropic) behavior of Tl-2212 samples.